The present invention relates to a novel method and apparatus for measuring the dynamic loading characteristics of a soil bed. More particularly, the invention concerns a method and apparatus for measuring, as a function of time, the resistance to penetration offered by a soil bed as the apparatus is fed into the soil at a constant rate. Even more particularly, the device measures and records the multidirectional forces exerted on the device as it is fed through several layers and types of soil in order to determine the types of soil involved, their resistance to loading forces, and their various thicknesses.
Uneven settlement of railroad track due to weak subgrade support is a well-known problem in the railroad industry. (See Chrismer et al., International Symposium on Cone Penetration Testing, Oct. 4-5, 1995, which is incorporated herein by reference.) In response to the settlement of track, railroads frequently implement remedial measures without knowing the cause of the track failure or the precise condition of the substructure beneath the track. Such remedial measures may include the placement of hot mix asphalt beneath the ballast layer--the top granular layer closest to the track--but above the subgrade, or lifting the track to tamp additional ballast under the railroad ties. If the cause of the failure is not properly diagnosed, an ineffective, and frequently costly, repair may be undertaken.
In addition, the maximum permissible axle load on North American railroads was increased in recent years from thirty-three to thirty-nine tons, thereby necessitating reinforcement of weak sections of track. Frequently, however, weak subgrade support does not manifest itself immediately in the form of track settling, and may not, as a result, be immediately evident.
Cone penetrometers have been employed for more than two decades in the testing of the characteristics of soil beds. Examples of two such devices are shown in U.S. Pat. No. 5,125,266, issued to Ingram et al. on Jun. 30, 1992, and U.S. Pat. No. 4,398,414, issued to MacGregor on Aug. 16, 1983, both of which are incorporated herein by reference. One problem associated with existing penetrometers has been the weakness of the cone used to initially penetrate the soil. The cones on previous devices have been constructed of materials and to specifications which make them ill-suited to withstand the repeated pushing through an extremely hard ballast layer that has been compacted by many years of train traffic. In some instances, the pushing force exerted on the cone upon entry into the ballast layer may approach 10,000 pounds per square inch (psi). Existing cones have been known to dent, bend, or otherwise fail when subjected to continually strenuous conditions over a period of time.
Some known devices have attempted to solve the problem of cone durability by employing a cone constructed of extremely hard materials. Many of these devices have also utilized cones with particularly thick outer layers or wide outer dimensions. These strategies have frequently yielded a more durable cone, but at the expense of measurement sensitivity. Cones having a thick outer layer and constructed of the hardest materials are unable to measure accurately the forces being exerted on them due to their decreased sensitivity to such forces.
Inaccuracies associated with many of the prior art devices can be attributed, in large part, to the devices' measurement of resistance at the tip of the cone only. Such devices have also produced inaccurate force measurements due to their inability to compensate for any displacement or shifting of the penetrometer device which may occur during insertion into the ground and may be caused by the presence of rocks or particularly hard soil in the path of the device. Further, methods used for testing the resistance of soil beds to loading forces have involved driving the penetrometer device into the ground at a variable rate of speed, thereby increasing the probability of inaccurate force measurements.
Existing devices that have attempted to increase accuracy of soil detection by measuring forces on both the tip of the cone and along the sides of the device have frequently employed a gauge-type layout to sense these forces and to transmit data associated with the forces experienced. The force measurements of these devices, however, have typically been subject to "cross-talk," i.e., interference between the gauges measuring the friction force and those measuring tip resistance. Such interference occurs due to the wiring together of the two sets of gauges such that the measurements from the two may become indistinguishable.